Generally, a computer system executes processing of various types using a plurality of computers. When building a computer system, in order to reduce the volume occupied by the device, sometimes virtual computers (hereinafter termed “virtual machines” (VMs)) are used rather than physical computers. A VM performs processing of various kinds by starting an operating system (hereinafter termed an “OS”) that is installed upon a virtual storage device (hereinafter termed a “virtual disk”). Such a virtual disk is implemented as a file in a file system. It is possible to create a plurality of virtual disks by creating a plurality of copies of a file system in which the file of a virtual disk for which the initial setup has been completed is stored. By doing this, a plurality of VMs can be created in a simple manner, already initially set up.
A technique is known for acquiring a snapshot of a file system (hereinafter termed an “SS”) that can be logically written (refer to PTL 1). According to this technique, when the file of a snapshot is being updated, differential data between the file before updating and the file after updating is stored in a different storage region from the snapshot. The destination for storage of this differential data is changed to a different storage region.